This invention relates generally to plasma generation techniques, particularly as used in a dry etching process in the fabrication of integrated semiconductor devices. In dry etching, process gases are supplied to a vacuum chamber and radio-frequency (rf) energy generates and sustains a plasma cloud within the chamber. Ions in the plasma cloud bombard a workpiece, usually in the form of a semiconductor wafer, which may be located in the chamber immediately adjacent to the plasma, or in a separate processing chamber into which ions from the plasma are drawn. The ions either etch the workpiece or assist the etching, and the etching process may be made selective by patterning a protective coating applied to the workpiece prior to etching.
In general, there are three types of plasma generation approaches: capacitive, inductive, and microwave. In the more conventional capacitive plasma approach, the plasma is formed between a pair of parallel plate electrodes, to which radio-frequency (rf) energy is applied, to one or both plates. A variant of the parallel plate approach is the magnetically enhanced reactive ion etch (MERIE) plasma generation apparatus, in which a magnetic field enhances the formation of ions in the plasma. Both the parallel plate configuration and the MERIE configuration typically use a single rf power generator. Inductive plasma generators use an inductive coil, either a planar coil, a cylindrical coil or any of various other types of coils to deliver rf power into the plasma chamber. A separate rf generator supplies energy to at least one plate electrode in the chamber, to control ion energy and direction.
Magnetic enhancement (MERIE) has been an extremely successful etching technique for sub-micron and sub-half-micron devices of various types. The process provides good etch profile control excellent reproducibility and process robustness. However, MERIE suffers from a drawback that results directly from the use of a magnetic field in the plasma. Under the influence of the magnetic field, electrons in the plasma tend to drift in one direction, while positive ions in the plasma tend to drift in the opposite direction. As a result, charge distribution in the plasma becomes laterally non-uniform and charge distribution on a wafer being etched is similarly non-uniform. This can cause wafer damage because a capacitor formed on the wafer, or any other device on the wafer having capacitive characteristics, can break down if the accumulated charge on the wafer reaches a threshold level. This device charging effect limits the magnetic field strength in some MERIE systems to about 30-50 gauss, and up to 200 gauss in other MERIE systems. Further, the etch process may need to be operated at a higher pressure regime than conventional reactive ion etch (RIE) systems, to minimize damage.
Another disadvantage of the MERIE process is that it has to be operated with a relatively high direct current (dc) bias between the parallel-plate electrodes. As a result, high-energy ions impinging on the waist may "splash" material onto sidewalls of structures, such as vias, being etched into the water. Thicker sidewalls formed during high-bias etching are, therefore, an undesired result of the MERLE process.
Inductively coupled plasma (ICP) reactors are generally considered to be a newer technology as compared to RIE or MERIE types of plasma reactors. The inductively coupled plasma uses an inductive rf coil to generate a higher density plasma (.gtoreq.5 mA/cm.sup.2) at a low pressure (&lt;30 mTorr). The reactors also have at least one electrode with the wafer placed on it and this electrode is capacitively powered by a second rf generator. The first inductive rf generator defines the density of the plasma and the second rf generator controls the ion energy.
The advantage of an inductively coupled plasma (ICP) reactor is to decouple ion energy control from ion density control, and a wider process latitude is usually obtained for metal and polysilicon etching because the chemical nature of the plasma and because of the independent controls of ion energy and ion density. The process "window" is, therefore, much wider and selectivity to the underlayer of the workpiece is significantly higher. However, selectivity to a photo-mask (i.e., the ability of the reactor to etch only selected areas t,f a workpiece through a mask having a photolithographically formed pattern) is usually lower than for a high pressure RIE or MERIE system.
For dielectric etching, the process is ion driven dominated etching, which is quite different from that of metal and polysilicon etching. Typical inductive or microwave generated plasma oxide etchers, operating at low pressure (approximately a few mTorr) have completely different characteristics from metal and polysilicon high density etchers. At low pressure, especially for high selectivity contact, via or self-aligned contact (SAC) process applications, by-product polymer is deposited over the entire inside chamber wall. The deposited polymer is not only a major particle source but significantly changes the baseline process and causes a high transient effect. The chamber wall can be heated to minimize deposition but the temperature has to be quite high (&gt;235.degree. C.) to be effective. Also, an inductively coupled plasma (ICP) requires high rf power input, which drives the chamber temperature high (.gtoreq.200.degree. C.-300.degree. C.). Process stability becomes extremely sensitive in this environment. A further disadvantage of ICP is that etch rate microloading is worse for ICP than for RIE or MERIE (i.e., the etch rate in a small space is usually lower than the etch rate in large spaces). Also, the sidewall passivation with ICP processing is usually thinner than with RIE or MERIE at higher pressure. As a result the profile tolerance for a long overetch is poor.
It will be appreciated from the foregoing that there is still a need for improvement over RIE, MERIE and ICP processes. Ideally, it would be desirable to provide a system with a medium density plasma (MDP) without all the drawbacks of the high density plasma, and yet retaining all the advantages of the traditional MERIE and RIE etchers. As further explained below, the present invention is directed to these ends.